1. Field of the Invention
The present invention relates to a driving apparatus for charge-coupled (hereinafter, a charge-coupled device is referred to as a CCD) image sensor, and more particularly to a system for precisely controlling the repetition frequency of transfer pulses.
2. Description of the Prior Art
The great part of the solid-state image sensors such as an interline type CCD area image sensor have a selected number of imaging elements in a horizontal line. Such selected number is suitable to be driven by the frequency which is integer times of the color subcarrier frequency in the NTSC color system. Typically, the driving frequency or the repetition frequency of charge transfer pulses is 7.16 MHz which is just double of the color subcarrier frequency which is 3.58 MHz. The number of the imaging elements in a horizontal line is 380. The main reasons of the selection of such driving frequency and the imaging element number are three technical factors. First, the present manufacturing limit of imaging element number is about four hundreds in a horizontal line. Second, the frequency of the reference signal in standard sync signal generator is 14.32 MHz, nowadays, which is easily divided to produce the driving frequency of 7.16 MHz by one frequency divider. Last, because the noise frequency generated from the one frequency divider is same as the reference frequency, the most harmful synchronous noise is not introduced into the driving apparatus of the CCD image sensor.
On the other hand, the reproduced picture picked up by the image sensor having 380 imaging elements in a horizontal line is not sufficient in views of resolusion and spurious image. Therefore, in order to reproduce a fine picture compatible with the image obtained by camera tubes, the increment of imaging element number is expected. However, so long as the frequency of the integer times of the color subcarrier frequency is employed as the driving frequency, the imaging element number should be sharply increased to the number such as 570 which is one and half times of 380 and 760 which is two times. Such sharp increment is very difficult in view of the present techniques.
However, if the driving frequency can be controlled to fit the increment of imaging element number, the element number can be increased without restraint. Measurement for controlling driving frequency, more particularly the phase of the driving frequency is a usage of a Phase-Locked-Loop (PLL) circuit or a keying controlled oscillator. The PLL circuit uses a serially connected frequency dividers, resulted in a generation of synchronous noises harmful for the reproduced picture. The keying controlled oscillator is unstable for variations in temperature and voltage, resulted in a vibration of reproduced picture.